A head-up display apparatus that is mounted on a mobile object, and projects an image onto a projection member to display the image as a virtual image visible to an occupant is conventionally known. A head-up display apparatus described in Patent literature 1 includes a light source portion that projects light from a light source, and a screen member having a diffusion portion that diffuses the light from the light source. The head-up display apparatus further includes a light-transmissive optical element that is disposed between the light source portion and the diffusion portion, has a first optical surface exposed to the light source portion and a second optical surface exposed to the diffusion portion, and transmits the light from the light source.
The inventor of the present application has found the following.
When the head-up display apparatus is mounted on the mobile object, the diffusion portion may need to be inclined due to mounting space and some reason. In such a case, when the light-transmissive optical element described in Patent literature 1 is adopted, a formed image may vary depending on sites of the image due to the inclination, causing variation in visibility. In addition, noise such as double image may be generated in an image by multiple reflections in the light-transmissive optical element.